Negative electrode for non-aqueous electrolyte secondary battery, method for producing the same, and non-aqueous electrolyte secondary battery

Abstract

This invention provides a negative electrode and a non-aqueous electrolyte secondary battery including the negative electrode. The negative electrode includes a Li-absorbing element as a negative electrode active material, is free from deformation, separation of the negative electrode active material layer from the negative electrode current collector, and deposition of lithium on the negative electrode current collector, and is excellent in cycle characteristic, large-current discharge characteristic, and low-temperature discharge characteristic.The negative electrode of this invention includes: a current collector having depressions and protrusions on a surface in the thickness direction thereof; and a negative electrode active material layer that includes a plurality of columns containing a negative electrode active material that absorbs and releases lithium ions, the columns being grown outwardly from the surface of the current collector. The negative electrode of this invention is characterized in that the columns are grown at an inclination angle relative to the direction perpendicular to the surface of the current collector, and that the inclination angle of the columns changes reversibly depending on absorption and release of lithium ions by the negative electrode active material.

Description

TECHNICAL FIELD [0001]The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery, a method for producing the same, and the non-aqueous electrolyte secondary battery.BACKGROUND ART [0002]With the remarkable proliferation of portable appliances such as personal computers and cellular phones, there is an increasing demand for batteries used as the power source for portable appliances. Among the batteries used in such applications, lithium ion secondary batteries have high energy density and excellent cycle characteristics, so demand for lithium ion secondary batteries is particularly increasing. Lithium ion secondary batteries usually include a positive electrode including a lithium-containing composite oxide, a negative electrode including lithium metal, a lithium alloy, or a negative electrode active material that absorbs and releases lithium ions, and a non-aqueous electrolyte. Recently, in an attempt to further improve the performance of l...

AI Technical Summary

Benefits of technology

[0034]According to the present invention, to ease stress created by the expansion of a high energy-density negative electrode active material, pores can be formed in the negative electrode active material layer. At the same time, by enlarging the contact area of the negative electrode current collector and the columns (negative electrode active material layer) compared to conventional art, sufficient bonding strength can be obtained. It is therefore possible to suppress deformation of the negative electrode such as distortion, wrinkles, or breakage. It is also possible to suppress separation of the negative electrode active material layer and partial exfoliation of the columns. As a result, the energy density, capacity retention rate, and charge / discharge cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0035]Also, according to the present invention, the inclination angle of the columns changes reversibly due to the absorption and release of lithium. During charging, in a cross-section of the columns in the thickness direction of the current collector, the angle formed between the center line of the columns in the grow direction thereof and the center line of the current collector in the longitudinal direction thereof increases, so that electrolyte is smoothly guided to the gaps among the columns. This facilitates the ion movement in an early stage of discharge and thus suppresses a drop in discharge voltage due to polarization, thereby enabling stable and sufficient large-current discharge and ensuring sufficient discharge capacity even in a low temperature range. Further, the battery safety also improves.

[0036]Also, according to the present invention, the inclination angle of the columns changes reversibly due to the absorption and release of lithium, and during discharging, the distance between the columns and the positive electrode active material layer decreases. Hence, in discharge, stable and sufficient large-current discharge is possible, and sufficient discharge capacity can be ensured even in a low temperature range. Further, the battery safety also improves.

Problems solved by technology

However, when Li-absorbing elements absorb and release lithium ions, they repeatedly expand and contract and undergo large volume changes, and this property of Li-absorbing elements limits their use as negative electrode active materials.

That is, when a Li-absorbing element is included in a negative electrode active material layer of a negative electrode, the negative electrode active material layer undergoes large volume changes due to the absorption and release of lithium ions, thereby creating a large stress.

Such a negative electrode is subject to deformation such as distortion, wrinkles, or breakage.

This may result in creation of space between the negative electrode and the separator or the negative electrode active material layer, uneven charge / discharge reaction, and degradation of cycle characteristics.

As a result, the lithium is not efficiently released from the negative electrode during discharging, the coulombic efficiency and the capacity retention rate lower, and eventually the cycle characteristics degrade.

Further, the battery safety also lowers.

However, since the area of the negative electrode current collector surface other than the area where the columns are formed is also covered with the thin film of the Li-absorbing element, expansion of the Li-absorbing element cannot be sufficiently eased.

Thus, the bonding strength between the negative electrode current collector and the columns becomes low, so that the columns become separated from the negative electrode current collector, which may result in degradation of charge / discharge characteristics.

Such a technical problem also exists in the negative electrodes of Patent Documents 3 to 6.

If columns grow from the surfaces of the depressions, the gaps between the columns become small, so that sufficient pores cannot be formed in the negative electrode active material layer.

Hence, the stress due to the expansion of the Li-absorbing element may not be sufficiently reduced.

This means that the lithium ions have to move a long distance.

Thus, particularly in discharge, large-current discharge cannot be sufficiently carried out, and the discharge capacity at low temperatures may become insufficient.

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